This invention relates to improvements in a high voltage ignition cable (hereinafter referred to as an "ignition cable") which is used to suppress radio interference generated by electrical ignition in an internal combustion engine, e.g., in a car, etc.
When conductive substances such as salts (e.g., for the prevention of freezing of roads in a cold district), sludge, etc., attach onto the external surface of a jacket of the ignition cable and the impedance thereof relative to the ground potential is lowered, the charging current flows out thereto according to the electrostatic capacity between a resistive-conductor core (hereinafter referred to as a "core", for simplicity) and the external surface of the jacket.
Therefore, as the electrostatic capacity increases, a reduction in the ignition voltage increases, resulting in poor ignition. In order to eliminate such poor ignition, it is necessary to use an ignition cable having as low an electrostatic capacity as 80 pF/m or less.
One way of lowering the electrostatic capacity is to increase the outer diameter of the ignition cable. However, increasing the outer diameter is not desirable, since the outer diameter of the ignition cable is usually about 7 or 8 mm, in that the ignition cable obtained can not be exhanged with conventional ones, and requires additional space.
In order to lower the electrostatic capacity while holding the outer diameter at a constant level, it is necessary to reduce the outer diameter of the core, and in order to lower the electrostatic capacity to the above-described level of 80 pF/m or less, it is necessary to reduce the outer diameter of the core to 1.2 mm or less.
By merely reducing the outer diameter of the core, however, the core will be cut off during the course of extrusion or vulcanization of the insulator, jacket, or the like, and thus it is not possible to produce, on a commercial scale, ignition cables which are sufficiently stabilized in high voltage withstanding ability, as in the case where glass fiber bundles are used as a tension member. The use of aromatic polyamide fiber bundles instead of the glass fiber bundle avoids the above-described defects but does not give a sufficient high voltage withstanding ability as described hereinafter. Furthermore, stabilized ability of the high voltage withstanding and problems such as difficulty in working of termination of the cable, etc., arise.